Sahih al-Bukhari 5920

Narrated Ubaidullah bin Hafs:that `Umar bin Nafi` told him that Nafi`, Maula `Abdullah had heard `Umar saying, "I heard Allah's Apostle forbidding Al-Qaza'." 'Ubaidullah added: I said, "What is Al-Qaza'?" 'Ubaidullah pointed (towards his head) to show us and added, "Nafi` said, 'It is when a boy has his head shaved leaving a tuft of hair here and a tuft of hair there." Ubaidullah pointed towards his forehead and the sides of his head. 'Ubaidullah was asked, "Does this apply to both girls and boys?" He said, "I don't know, but Nafi` said, 'The boy.'" 'Ubaidullah added, "I asked Nafi` again, and he said, 'As for leaving hair on the temples and the back part of the boy's head, there is no harm, but Al-Qaza' is to leave a tuft of hair on his forehead unshaved while there is no hair on the rest of his head, and also to leave hair on either side of his head